A conventional IECF device 10 is shown at FIG. 1. An electrical potential is created by placing a small inner grid 20 within a larger, concentric outer grid 30, and supplying a high voltage feed 50 to the inner grid. Usually, the inner grid 20 is the negative electrode, at high voltage, and the outer grid 30 is the positive electrode, at ground, to protect operational personnel from the high voltage. The outer grid 30 may be a spherical outer chamber. When the outer grid 30 is a solid surface sphere, it may be also act as a vacuum chamber. If the outer grid 30 is a non-solid surface sphere, the outer grid 30 is contained in a separate vacuum chamber (not shown).
The inner grid 20 is usually in the form of a spherical wire grid. The inner grid 20 is typically supported by a ceramic insulating stalk 40. The ceramic stalk 40 is a conduit for a high voltage feed 50 to pass through and be connected to the inner grid 20.
A vacuum is formed within the outer grid 30 by a vacuum system 60. The vacuum may be formed within a separate vacuum chamber if provided. The outer grid 30 or vacuum chamber is filled with a fill gas from a fill gas source 70.
An IECF device is normally operated at a reduced pressure on the order of 10-30 microns absolute (0.01-0.03 torr) with a fill gas that is typically, but not limited to, deuterium, tritium or deuterium/tritium mix. The fill gas provides a source of ions, which are formed in situ when the potential is applied, so that any gas capable of ionization can be used. Ions can also be introduced into the device from an ion gun, particle beam or any other known ion source that may inject the ions into a neutrally charged fill gas.
When the apparatus is placed under vacuum and a high voltage feed 50 is applied to the inner grid 20, ions are formed from the fill gas by the flow of electrons between the outer grid 30 and the inner grid 20. A voltage of 10 KV to 1 MV DC may be applied to the inner grid 20, with 20 KV to 200 KV DC more commonly applied. An electrical potential is thus created between the outer grid 30 and the inner grid 20. Fill gas ions are then accelerated from the space approximately between the outer grid 30 and the inner grid 20 towards the inner grid 20. These ions either pass through the inner grid 20 towards the center of the device or impact the wire structure of the inner grid 20.
The inner grid 20 is the subject of much study since it must be designed to provide a uniform accelerating potential for ions but must also be designed to minimize the obstruction for ions to pass through to reach the center of the device. The inner grid 30, at high voltage, provides the potential through which positive ions such as deuterium ions are accelerated toward the center of the IECF device.
The inner grid 20 is usually in the form of a spherical wire grid structure, which has a surface that is substantially empty space between the wires. This empty space allows ions to pass through towards the center of the device, allowing those ions to collide with other ions that are similarly being accelerated towards the device center or with other particles simply present in the path of the accelerated ions. In this manner, ions from the space between the electrodes are accelerated toward this inner grid 20. These accelerated ions gain enough energy, typically more than 20 KeV under the proper amount of potential, and fuse with other ions, neutral atoms and/or molecules, releasing nuclear fusion energy.
Two limitations are inherent to a wire designed inner grid. The first limitation is that the maximum number of ions do not reach the center of the apparatus because a certain amount of ions impact upon the wire of the inner grid structure and terminate upon the surface of the wire. These ions cannot contribute to collisions. The second limitation is that the ions that impact the wire of the inner grid cause damage to the grid and affect the temperature at which the device is operated. The impacting ions transfer their kinetic energy to the wire material and increase the temperature of the wire material.
The first limitation can be minimized by maximizing the openings of the inner wire grid to allow ions to pass through. However, maximizing the openings decreases the uniformity of the potential field created by the grid. For best accelerating performance, a solid inner sphere would provide for the most uniform acceleration, but this configuration would not allow any ions to pass through to the center of the device. To maximize the amount of ions reaching the center of the device, the amount of wire material forming the inner wire grid may be reduced, but this decreases the uniformity of the field close to the inner grid.
Thus, with a conventional open wire inner grid, there is always a trade-off between uniformity of acceleration potential and the amount of ions reaching the center of the device. A compromise that is normally used is an inner grid structure formed of an open wire inner grid of 0.5 to 3 inch diameter wire rings. These rings, usually 4 or more, are spot welded to each other to form a sphere of mostly open space. A wire diameter of 0.020 inch is frequently used to form the wire rings.
From a perspective outside the surface of the inner wire grid, the grid electrically looks like a spherical point charge to accelerating ions and is a good alternative to a solid inner spherical electrode. However, a portion of the ions will still be blocked from passing through the inner grid to the center of the device since some portion of the ions will still impinge upon the wires forming the inner wire.
The second problem present with any wire inner grid of any geometry is that ions impinging upon the grid cause heating of the grid, even destruction of the wire grid from melting under certain operating conditions. The ions accelerating toward the inner grid have a great amount of kinetic energy from the accelerating potential. This kinetic energy is dissipated as heat when ions impact the inner wire grid. Impacting ions cause the wire to heat and glow from incandescence, and melt the wire grid at all but the lowest operating power. The use of tungsten wire or the like can increase operating power levels, but undesirable melting of the inner grid wire will still occur at higher power levels of approximately greater than 2 kW.